1. Field of the Invention
The present invention relates to long-life cryogenic coolers. More specifically, the present invention relates to position sensors used in linear-oscillating, closed-cycle cryogenic coolers.
2. Description of the Related Art
For certain applications such as space infrared sensor systems, a cryogenic cooling subsystem is required to achieve improved sensor performance. Numerous types of cryogenic cooling subsystems currently exist, each having relatively strong and weak attributes relative to the other types. Some space cryocoolers, for example, offer efficiency, operational flexibility and vibration performance at the expense of increased mass and volume relative to other available systems.
Long-life linear-oscillating cryogenic coolers are often designed for lifetimes of well over 10 years under continuous operation. These cryocoolers are not only expected to reliably provide effective cooling for many consecutive years, but are expected to very precisely control the temperature of their cold sinks by actively and precisely varying the stroke characteristics of their internal operating elements. These long-life cryocooler systems therefore include active reciprocating-element position feedback. The stroke amplitude and offset of the various reciprocating elements must be monitored and controlled so that the moving elements do not overstroke and therefore physically impact other parts of the cooler during operation. Such an overstroke or impact can mechanically damage the moving elements and their flexure suspension system, as well as create large amounts of vibration and shock forces that many systems (to which the cryocoolers are attached) are simply unable to tolerate.
The long-life nature of these designs requires that these position measurements be zero-contact so that no friction exists and no contaminants are generated. The sensors used to take these measurements are often a significant portion of the total cryocooler system size, require significant amounts of support electronics, and are usually very expensive.
A variety of sensors have been used in the past to make zero-contact measurements of oscillating element stroke amplitude and phase. Most notably, Linear-Variable Differential Transformer (LVDTs) sensors, capacitive sensors, and eddy-current sensors have been used. Generally the sensors provide accurate, continuous position feedback with zero contact. The continuous-feedback nature of these devices implies significant mechanical and electronic complexity; LVDT sensors are very large, expensive, and require significant drive and demodulation circuitry. While capacitive and eddy-current sensors are somewhat smaller, they require specialized demodulation circuitry and are also expensive in small quantities.
The inclusion of position feedback sensors in the various cryocooler mechanisms has the effect of increasing the total cryocooler package size and mass.
Hence, a need remains in the art for a system or method for sensing the position of moving components of long life cryogenic coolers.